It is well known in the art that a watch case enclosing an electronic movement can be filled with a clear dielectric liquid, typically a silicone oil or a perfluorinated fluid. The dielectric liquid improves water resistance of the watch, prevents condensation of water inside the case, ensures continuous lubrication of the movement, and enhances readability of the dial by reducing undesirable refractions of light.
One challenge in designing a liquid-filled watch is compensating for differential thermal expansion. The thermal expansion coefficients of the known dielectric liquids are significantly larger than those of the known materials used in construction of watch cases, including steels, ceramics, and plastics. Since liquids are incompressible, their expansion inside a confined volume can exert tremendous pressure that is adequate to rupture or displace a watch's crystal. Several methods are known in the art for accommodating thermal expansion in a liquid-filled watch, thereby enabling a wide range of operating temperatures.
For example, U.S. Pat. No. 4,080,781 discloses a watch filled with a liquid and a device for accommodating thermal expansion. In one embodiment, the case is designed with a movable piston in the side wall. The piston extends and retracts to follow changes in the internal liquid volume. In a second embodiment, there is an elastically deformable diaphragm built into a wall of the case. The diaphragm allows the liquid to expand and contract. Both embodiments add substantial mechanical complexity to the case design, resulting in additional costs of manufacturing.
U.S. Pat. No. 4,459,039 discloses a watch filled with a liquid that has one internal enclosure with a deformable wall. The internal enclosure is filled with a gas, and compensates for thermal expansion of the liquid. Three embodiments are disclosed for the internal enclosure: a diaphragm sealed to the interior of the case that contains a fixed quantity of gas; a gas-filled bladder made entirely of flexible material; and a rigid tube having one closed end and a movable flexible plug that holds a fixed quantity of gas inside the tube. A disadvantage of a single interior enclosure is that a leak in the flexible material will allow all the gas to escape from the enclosure and form a large bubble in the liquid. Another disadvantage is the complexity and cost of fabricating the internal gas-filled enclosure.
International Patent Publication No. WO 96/35978 discloses a watch filled with a dielectric fluid in which there is a gas bubble serving to compensate for the differential coefficients of thermal expansion between the liquid and case. One disadvantage is that the gas bubble can be seen floating freely under the crystal. The bubble can be aesthetically objectionable to some users. Another disadvantage is that the procedure for injecting the liquid requires tight control in order to target a specific size of the gas bubble.
German Patent Publication No. DE 19647439 discloses a watch filled with a liquid where the case is designed with a movable piston in the case back. The piston extends and retracts to follow changes in the internal liquid volume. By locating the piston in the case back instead of the side wall of the case, there is room for a piston of significantly larger diameter. The larger diameter bore serves to reduce the travel distance of the piston as it responds to changes in liquid volume. The piston feature adds design complexity and manufacturing cost.
French Patent Publication No. FR 2752959 discloses a watch filled with a liquid where the base of the case is provided with a deformable membrane that is protected by a rigid external wall. In one embodiment, the membrane is constructed of thin stainless steel with circular corrugations to allow for expansion. Small openings in the rigid external wall allow pressure equalization between the membrane and the exterior environment. In addition to the complexity and high manufacturing cost of this design, the space between the membrane and the rigid external wall fills with water when the watch is immersed. After exiting the water, the residual water in the space gradually drains onto the wrist of the user.
International Patent Publication No. WO 03/083586 discloses a watch filled with a liquid where the base of the case at least partly comprises an elastic membrane, and an adjacent rigid element to limit deformation of the membrane toward the case interior. The membrane flexes in order to accommodate thermal expansion of the liquid. The membrane feature adds complexity to the case design, resulting in a higher cost of manufacturing.
The invention described herein seeks to provide an improved thermal expansion compensator for liquid-filled watches.